2 Answers
2

RSS is a tool. It's used by subscribers to quickly/easily see when a site they're interested in is updated/adds new content. If you think your feed subscribers would like to be notified of such updates and might be interested in viewing it (possibly even for the first time), then sure add it to the feed. If you think most of your subscribers wouldn't care and would just be annoyed by frequent minor updates clogging their feed, then don't include them.

For example, if I make a minor edit to one of my past blog entries from a month ago (fix a typo, a dead URL, etc.), I'm not going to add that in the newsfeed. However, if I revisit an old article from 2-3 years ago, totally revising it to update it with current info or appending an entirely new section to follow up on an old topic, then I'd likely want it included in the feed.

That said, most CMSes don't include updates in newsfeeds, so if it's a significant update, it's sometimes better to just make a new post and link to old one in the intro or header.

@Jack: If you have your feed syndicated on other sites then it could I suppose. The updated page would receive the backlinks while the post that was pushed off of the feed wouldn't. But that's about it.
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Lèse majestéApr 9 '12 at 9:13

Whether you "should" is your own decision. But there are different ways to approach it if you do.

Some sites just edit the original post directly, and tack something like "[UPDATED]" on to the headline. [example at VentureBeat] This has two potential effects, which cover everyone:

If someone's feedreader hadn't already fetched the item, then it will show up with the updated flag on the headline, indicating to the person they should click through for newer information.

If the reader did already fetch the item, then they'll see the flag when they arrive.

In my opinion, this arrangement is most effective when your feed only offers partial content, since it requires clicking through for the new information, and especially in the second case.

Another approach is to just post a new item, with backlinks to the original content, one example being Jason Kottke's update posts. This clearly alerts everyone that new content's been posted without flooding their feed with new items.

It should also be noted that besides CMS applications likely not updating feed items eg. by updating their timestamps(though they can still be made to via templating, etc), some feedreaders also choose not to do anything with updated posts. For example, FeedDemon used to have a feature that would re-display updated posts, which was eventually removed because pretty much everyone hated it. It was generally just confusing because it just seemed that posts were reappearing for no good reason in many cases, eg. minor undetectable edits, especially for users who weren't aware of the feature.

Because of that, whichever method you go with should still have some sort of clear indicator of the update rather than expect readers to just find the changes.